Method and apparatus for producing charcoal

ABSTRACT

A method for producing charcoal consisting of introducing the material to be carbonized into, through, and out of a single chamber in which it is heated to a carbonizing temperature by the combustion of gases emitted by the carbonizing of previously inserted material, the gases being intermixed for combustion with air also injected into the chamber, but the air and gas being directed and controlled in the chamber in such a manner that the carbonizing material is never exposed to sufficient oxygen to support free combustion thereof, and an apparatus capable of carrying out this method.

United States Patent 14 1 Aug. 26, 1975 Smith METHOD AND APPARATUS FOR PRODUCING CHARCOAL [76] Inventor: David E. Smith, Rt. 2, Rolla, Mo.

[22] Filed: June 4, 1973 [21] Appl. No.: 366,785

[52] US. Cl. 201/32; 201/37; 202/131; 202/216 [51] Int. Cl....C10B47/20;C|0B49/06;Cl0B49/18; C108 H10 [58] Field of Search 201/32, 33, 36, 37, 27, 201/29, 15; 202/216, 218,131,136; 110/14 [56] References Cited UNITED STATES PATENTS 1,458,410 6/1923 Hiller 202/262 X 1,564,730 12/1925 Walden 201/27 1,957,364 5/1934 Stafford 201/27 2,813,822 11/1957 Collier 201127 3,306,237 2/1967 Ransom 1. 110/14 Bamebey 201/27 Bielski et a1. .1 202/136 X 5 7 1 ABSTRACT A method for producing charcoal consisting of introducing the material to be carbonized into, through, and out of a single chamber in which it is heated to a carbonizing temperature by the combustion of gases emitted by the carbonizing of previously inserted material, the gases being intermixed for combustion with air also injected into the chamber, but the air and gas being directed and controlled in the chamber in such a manner that the carbonizing material is never exposed to sufiicient oxygen to support free combustion thereof, and an apparatus capable of carrying out this method.

2 Claims, 5 Drawing Figures PATENTED AUBZB I875 SHEET 1 OF 5 PATENTEB M182 6 [975 sum u BF 5 PATENTEU M1826 I575 SHEET 5 BF 5 METHOD AND APPARATUS FOR PRODUCING CHARCOAL This invention relates to new and useful improvements in methods and apparatus for producing charcoal. The invention has been conceived and developed particularly for the production of charcoal from wood chips, but it will be readily apparent that the principles thereof are readily applicable to other materials.

The principal objects of the present invention are the provision of a method and apparatus which provides a continuous operation, with fresh wood chips being continuously added and completed charcoal being continuously removed, as distinguished from *batch" methods in which a quantity of material to be carbonized is sealed in a kiln or the like, heated to carbonizing temperature by external fuel, allowed to Carbonize and then c ol, and finally removed to permit recharging of the kiln with a fresh batch; which permits the use of heat provided by combustion of combustible hydrocarbon gases emitted from carbonizing wood to elevate freshly added wood chips to carbonizing temperature, thereby eliminating any requirement for external fuel once the process has been initiated; and which permits the entire process to be carried out in a single chamber or retort, with the air introduced for combustion of the wood gases being restrained from contact with the carbonizing wood by controlled movement of the wood, gas and air within the chamber.

Another object is the provision of a method and apparatus of the character described which is clean" in operation, that is, it discharges so little smoke or other objectionable fumes to the atmosphere as to be acceptable under present strict environmental regulations. Such regulations are particularly strict in cities or other densely populated areas, and have in the past often necessitated location of charcoalling operations in remote areas.

A further object is the provision of a method and apparatus of the character described which is capable of producing large quantities of useful heat as a byproduct, which may be utilized in many different ways,

Other objects are simplicity and economy of both the method and the apparatus. and efficiency and depend ability of operation.

With these objects in view, as well as other objects which will appear in the course of the specification, ref erence will be had to the accompanying drawing, wherein:

FIG. I is a side elevational view, partially broken away, of an apparatus embodying the present invention, and capable of carrying out the method of the invention,

FIG, 2 is an enlarged sectional view taken on line llll of FIG 1,

FIG. 3 is a fragmentary sectional view taken on line IIIIII of FIG. 2,

FIG. 4 is an enlarged sectional view taken on line lV-lV of FIG. I, and

FIG. 5 is a fragmentary sectional view taken on line V-V of FIG. 4.

The process of producing charcoal from wood constitutes the destructive distallation of said wood, and is performed by heating the wood to drive therefrom first the free moisture and then the volatile hydrocarbon constituent elements of the wood, reducing it ideally to carbon, plus the small proportion of incombustible ingrcdients thereof. 1 he disposition of the volatile hydrocarbons is called carbonization. The carbonization proceeds spontaneously once it is initiated, initiation requiring that its temperature be elevated to about 550 Fahr. It is an exothermic chemical reaction, releasing heat equal to about 6% of that obtainable by complete combustion of the wood, and elevates the temperature of the wood to about 850 Fahr. It is also accompanied by the release of large quantities of hydrocarbon wood gas, which is combustible if intermixed with air to supply the necessary oxygen. However, since the wood temperature during carbonization is well above its ignition temperature, the process must be carried out in the absence of sufficient oxygen to support combustion, in order to avoid burning of the wood to ash. In older charcoalling methods, the heat necessary to dry and heat the wood to carbonization temperature was sup plied entirely by an external commercial fuel such as oil or gas, which made such processes relatively expensive. Also, the heat produced by the carbonization reaction was ordinarily eventually dissipated to the atmosphere as a waste product. Also, the wood gases were ordi narily burned off as a waste product, but this produced large quantities of smoke and noxious fumes, which were always objectionable, and which are now more and more strictly prohibited by environmental regulations. Such gases could be processed to produce by products such as ascetic acid, methanol, oils and tars, etc., but such products are presently more cheaply available from other sources, and the commercial value of wood gases for these purposes is negligible.

The method contemplated by the present invention comprises the following steps:

l. Introducing wood in the form of chips or the like into one end of an elongated chamber or retort, conveying it through said retort, and removing it from the opposite end of said retort, in a continu ous movement.

2. Burning an external fuel in the entry end of the retort for a time sufficient to dry and elevate a portion of the wood chips to carbonization temperature, whereby production of heat and wood gas by the carbonization reaction is commenced, after which the fuel supply is terminated.

3. Introducing air in a controlled amount into the retort to cause combustion of the wood gases pro duced to generate additional heat, the carbonization heat and heat produced by the combustion of the gas being utilized to dry and heat freshly introduced wood to carbonization temperature.

4. Controlling and directing movement of the wood, gas and air in such a manner that while the wood is thoroughly exposed to the available heat, it is not exposed to sufficient oxygen to support combustion thereof.

Other steps performed by the apparatus to be described, such as the cooling and storage of the charcoal produced, and the disposition of any unburned combustible material emerging from the retort, are considered merely ancillary to the process performed in the retort, and are not here listed.

The process as just described provides continuous, high speed production of high quality charcoal in a single chamber or retort. The wood gas, which is an inherent by-product of the carbonization reaction, is utilized to produce the heat necessary to make the process continuous after the initial starter flame is shut off. Moreover, although the wood gas is mixed with air and burned within the carbonization retort, the air does not reach the wood to cause burning and reduction thereof to ash. A pilot plant constructed to perform this method has consistently produced charcoal with less than 3% ash, the amount permitted in high-grade industrial charcoal. In other usages, a much higher ash content is permitted. It should be noted that while carbonizing wood does release heat, such heat is not sufficient to dry and heat the freshly added wood to carbonization temperature, and that the reaction would be snuffed out were it not for the additional heat supplied by combustion of the wood gas. The initial drying of the wood, for example, often requires several times the heat required to raise the sensible temperature of the wood from atmospheric to carbonization temperature.

In the drawing, which illustrates an apparatus capable of carrying out the method described above, like reference numerals apply to similar parts throughout the several views, and the numeral 2 applies generally to a retort which constitutes a large cylindrical vessel with its axis inclined slightly from horizontal, about which a pair of track members 4 are concentrically affixed as by spoke members 6. Each track member is supported by a pair of rollers 8 disposed respectively at opposite sides of the midline of the retort, each roller being carried rotatably by a ground-supported pillow block 10. At least one pair of said rollers is driven rotatably in the same direction by an electric motor 12 also mounted on a ground-supported pillow block 14. As best shown in FIG. 4, sprockets l6 fixed on the rotor of said motor are operatively connected by sprocket chains 18 to sprockets 20 fixed respectively on the axles of two of rollers 8. Thus the retort is turned slowly (about 2 or 3 rpm) about its axis in the direction of arrow 22 in FIGS. 2 and 4.

At the wood-entry end of the retort, which is its higher end, the retort is provided with an end wall 24 having formed therein a central circular aperture 26 into which extends a stationary, horizontal flue pipe 28 which is interconnected at its opposite end into a vertical, ground-supported stack 30 which may be of any necessary or desired height. Below the level of flue pipe 28, stack 30 is provided with a plurality of air-admitting apertures 32 (see FIG. 3) spaced angularly thereabout, each aperture being provided with a hinged closure door 34 operable to regulate the amount of air introduced by its associated aperture. Disposed above flue pipe 28 is a hopper 36, or other storage or conveying means, operable to deposit wood chips or the like at a pre-determined rate, regulated for example by a "damper valve 39, into a chute 38 which extends in sealed relation through the top wall of the flue pipe, then into retort 2 through the inner end of said flue pipe, and deposits said chips in the lower portion of the retort, at the descending" side of the midline thereof. The entire inner surface of the retort, including its end walls, is provided with a thick lining 40 of ceramic insu lating material. Also extending through a wall of flue pipe 28 is a fuel pipe 42 for gas or the like, controlled by valve 44, and terminating in a burner nozzle 46 (see FIG. 3) directed into the retort through the inner end of the flue pipe.

At the opposite or lower end of the retort, said retort is provided with an end Wall 48 provided with a central circular aperture 50 through which extends an air pipe 52. Externally of the retort, said air pipe is intercon nccted to an air blower 54 driven by an electric motor 56, and operable to draw a pre-determined amount of atmospheric air, regulated for example by a damper 58, through an intake pipe 60, and to deliver said air to the interior of the retort. It will be understood that the blower and air pipe are rigidly supported exteriorly of the retort by any suitable means, not shown. Interiorly of the retort, air pipe 52, has a first section 52A extending downwardly adjacent end wall 48, a second section 52B extending parallel to the retort axis but adjacent the bottom of the retort, a third section 52C extending upwardly adjacent end wall 24, and an open spout 52D opening into flue pipe 28, as best shown in FIG. 3. Spaced along section 52B, a series of holes 62 are formed in the air pipe, all of said holes being directed identically, slightly above horizontal, to cause circulation of air and gas in the retort in a direction opposite to the rotation of the retort itself, as indicated by arrows 64 in FIG. 2. While the air pipe is provided with a coating 66 of insulating material, it is located in a very hot zone, and might become overheated were it not for the terminal spout 52D thereof, which permits a larger quantity of air to be delivered by the blower to the pipe than is actually required at holes 62 thereof. This larger air volume has a cooling effect, and prevents overheating of the air pipe.

At the lower end of the retort, adjacent end wall 48 thereof, a plurality of scoops 68, each being elongated and of U-shaped cross-sectional contour, are fixed to the inner surface of the cylindrical retort wall at regular angular intervals, and project through and inwardly from the refractory lining 40 of the retort. They are so arranged as to open upwardly when on the upwardly moving side of the retort, and at the bottom of the retort they pass between end wall 48 and section 52A of the air pipe (see FIG. 5). As will be described, these scoops transport finished charcoal upwardly from a bed of charcoal in the lower portion of the retort, each scoop, as it approaches the top of the retort, depositing its contents in a hopper 70 disposed above air pipe 52 adjacent the inner surface of retort end wall 48. Said hopper directs the charcoal through a downwardly in clined tubular chute 72 which is inclined and passes downwardly through the top wall of air pipe 52 inside of the retort, through the bottom wall of the air pipe ex ternally of the retort, and is connected in sealed relation to a horizontal auger tube assembly 74, at one end of the latter. Said assembly includes an inner tube 76 into which chute 72 is sealed, and in which an auger 78 is operatively mounted, and driven by an electric motor 80, and an outer tube 82 spaced outwardly from tube 76 to form a chamber 84 through which cooling water is circulated by any suitable means through hose connections 86. At the end of the assembly opposite from chute 72, auger tube 76 is connected, as at 88, to a storage container for the charcoal. As best shown in FIG. 5, a flexible sealing strip 92 encircles air pipe 52 adjacent the external surface of retort end wall 48, being affixed to said air pipe as by rivets 94, and having a wiping contact with end wall 48, whereby to prevent air from entering the retort through aperture 50 around the air pipe, for a reason to be described.

In operation, wood chips or the like are allowed to enter the retort through chute 38 from hopper 36, at a rate regulated by damper 39, with the hopper being rotated in the direction of arrow 22 by operation of motor 12. Due to this retort rotation, the wood chips collect in a bed 96 at the bottom of the retort, and the bed tends to ride up the ascending side of the retort, as indicated in FIGS. 2 and 4. It of course can rise in this manner only until its exposed surface reaches the normal angle of repose" of the chip material, which is about 45 degrees. whereupon the chips begin to tumble and slide in a circulatory path within the bed, said path being indicated by arrows 98 in FIG. 2. Due to this tumbling action, the chips throughout the mass of the bed are repeatedly exposed both to the hot gases which as will appear are present in the retort, and to the liner 40 of the retort. Due to the inclination of the retort, the tumbling action of the chips causes them to advance slowly toward the opposite or lower end of the retort, until eventually the bed 96 will be of substantially uniform depth along the entire length of said retort. The feed rate of the chips from hopper 36 is so regulated, in relation to the degree of tilt of the retort, that the chip bed 96 will be of such depth that it engages the retort wall only substantially over the lower quadrant thereof. at the ascending side thereof, as shown in FIGS. 2 and 4.

After insertion of the wood chips into the retort, an external fuel such as gas is injected through nozzle 46 and burned to provide drying, heating, and the com mencement of either carbonization or actual combustion of some of the chips. Carbonization rather than combustion can be encouraged by leaving blower 54 shut off at this time. so that there will be relatively little air within the retort to support actual combustion. In any event. at least some of the chips will be started into a carbonization reaction even though others may be in full combustion. The fuel supply to nozzle 46 may be shut off at valve 44, and the process will thereafter be selfsustaining. The retort could also be heated externally to initiate carbonization. Blower 54 is then set in operation.

Operation of blower 54 introduces air into the retort through holes 62 of air pipe section 52B, which are spaced along substantially the entire length of the retort, in a direction indicated by arrow 100 in FIG. 2, setting up a clockwise circulation of air and gases in the retort as indicated by arrows 64, opposite to the counter-clockwise rotation of the retort itself. In passing over the chip bed 96, this circulation entrains the combustible wood gases being produced by the carbonization reaction occurring within the bed, and these wood gases are then intermixed with fresh air emerging from holes 62, principally in the lower quadrant of the retort at the descending side thereof. The wood gas and spent gases in which it is entrained are of course extremely hot, and the air in pipe 528 is pre-heated by the presence ofthe pipe in the circulating zone ofthe hot gases, and therefore the wood gases burn freely when intermixed with the air. The principal zone of combustion or fireball. is indicated at 102 in FIG. 2. The quantity of air introduced is much less than that required for full combustion of the wood gas, since full combustion would produce much more heat than is required to dry and heat freshly inserted wood chips to carbonization temperature. Nearly all of the available oxygen of the air introduced will be consumed in the fireball zone, and virtually none will remain by the time the hot gases again circulate around the retort and flow over the chip bed. Instead, the gases circulating over the bed will consist virtually entirely of spent air, unburned or only partially burned wood gases. and of course smoke.

These gases will be amply hot to dry and heat the chips to carbonization temperature as and after they are inserted, while the heat produced by the exothermic character of the carbonization reaction already occurring in previously inserted chips would not, but contain virtually no free oxygen, which would cause combustion of the chips. Of course, fireball 102 also heats the retort liner 40 to a very high temperature, and the liner is also an important factor in supplying heat to the chip bed. The previously described tumbling circulation of the chips within the bed insures that all of the chips will be thoroughly and intimately exposed both to the circulating gases and also to the liner.

If the length of the retort has been properly selected relative to its degree of inclination, carbonization of the chips to charcoal will be completed by the time they reach the lower end of the retort. At this point, as shown in FIG. 4, scoops 68 pass upwardly through the bed in succession, each scoop trapping a portion of the finished charcoal, elevating it, and dropping it into hopper 70 for conveyance through chute 72 and auger tube 76, in which it is water cooled, to storage container 90. The current of cool air in air pipe 52, which chute 72 traverses, also assists in the cooling of the finished charcoal. As previously mentioned, it is preferable that storage container be sealed. since unless auger-tube 76 is impractically long, there might still be hot spots" in the charcoal, which could cause combustion if the charcoal were immediately exposed to air. Scoops 68 must of course be of sufficient number and capacity to remove charcoal from the retort as fast as wood chips are added, despite the slow rotational speed of the retort. They may in fact be provided with a still greater removal capacity, since this would not adversely affect operation, and would permit adjustment of the rate of introduction of chips with no possibility of pile-up of charcoal at the lower end. Adjustment to a greater rate of introduction of chips may be possible, for example, when the chips are relatively dry, and hence require less time, and a smaller proportion of the retort length, for the drying phase of the operation. The desirability of air seal 92 may now be appreciated. While the mere admission of air to the retort is not in itself objectionable, since air is being introduced by blower 54 at all times, the admission of air at this point could provide oxygen for combustion of the hot charcoal which might reach the coal as it is elevated and dropped by the scoops.

The spent air, unburned or partially burned hydro carbon wood gases. and smoke, emerge from the retort through flue pipe 28 to stack 30, where their high temperature creates a strong updraft. In passing through the flue pipe, which chute 38 traverses, they also assist in pre-heating the chips as they pass through the chute. If allowed to escape to the atmosphere in the condition they leave the retort, they would constitute an objectionable environmental threat, with heavy smoke representing unburned particulate matter, and noxious fumes. The present apparatus overcomes this difficulty by admitting air to the stack below flue pipe 28, through door apertures 32, as regulated by doors 34. The smoke and gases in the flue contain much matter still combustible and still hot enough to burn, but unburned due to lack of oxygen. Air admitted by doors 34 therefore causes ignition of the stack gases. creating a very hot flame which consumes virtually all combustible material still remaining. Preferably all of doors 34 are hinged at the same side, so that if they are opened slightly, the air is admitted with a swirling motion into the stack. This creates a vortex action in the stack, which promotes a fuller degree of combustion by retaining the gases in the stack for a longer time period. The afterburner provided by admission of air to the stack provides a clean" emission from the stack, which is virtually free of all smoke or other objectionable fumes or gases, While not considered to be properly a portion of the present invention, it is of interest that heat released by the stack combustion is quite large in amount, about 2,000 B.t.u. per pound of wood chips carbonized. With a retort of the size contemplated and tested, (about 7V2 ft. in diameter and 16 ft. long), which is designed to process about 5,000 lbs. of chips per hour, the heat released in the stack thus amounts to about 10,000,000 B.t.u./hr. and this heat could be utilized in many valuable applications.

It will of course be apparent that when operation is first started, and until the heat and flow conditions of wood chips, air and gas throughout the retort have become stabilized, a quantity of completely or only partially carbonized chips, and charcoal of low quality, may be produced. This initial output may be discarded as waste, or preferably recycled to hopper 36 for reprocessing.

While I have shown and described a specific embodiment of my method and apparatus, it will be readily apparent that many minor changes of both the method, and also of the structure and operation of the apparatus, may be made without departing from the spirit of the invention.

What I claim as new and desire to protect by Letters Patent is:

l. A method of producing charcoal from wood or the like in small pieces consisting of the following steps:

a. continuously introducing said wood into the higher end of an elongated, slightly inclined retort which is inclined downwardly toward its opposite or exit end,

b. rotating the retort about its longitudinal axis, whereby the wood pieces are constantly tumbled and advanced toward the lower end of the retort along the length thereof, in the form of a bed only partially filling said retort,

c. heating said wood by combustion of an external fuel long enough only to dry and initiate carbonization of a portion of said wood, whereby a combustible wood gas is driven from said wood, and

d. introducing air tangentially into a peripheral por tion of the retort not occupied by the wood bed, in

a direction opposite to the rotation of the retort and at a point angularly remote from the bed, considered in the direction of circulation of the air, whereby wood gas driven off is entrained in the circulation, mixed with the air, and burned to the extent permitted by the available oxygen of the admitted air, before the circulating gases again engage said bed, the heat of said combustion serving to dry subsequently added wood and bring it to carbonization temperature, and the tumbling of the wood pieces in the bed serving to expose the entire mass of said bed to the heat of said combustion, and

e. removing the resulting charcoal from the lower end of said retort.

2. An apparatus for producing charcoal from wood or the like in small pieces comprising:

a. an elongated retort the axis of which is inclined slightly from horizontal whereby to have upper and lower ends,

b. means operable to introduce said wood into the upper end of said retort at a regulated rate,

c. means operable to rotate said retort about its longitudinal axis at a rate sufficiently slow that said wood gathers in a bed in the lower portion of said retort, said bed tending to rise along the ascending side of the retort until gravity causes the wood pieces to tumble, said bed also being moved along the length of said retort as a result of said tumbling and the inclination of said retort,

d. means operable to burn an external fuel in or adjacent said retort long enough only to dry and bring to carbonization temperature a portion of said wood, whereby carbonization is initiated and a combustible wood gas emanates from said wood, and

e. means operable to inject air in regulated amounts tangentially into a lower peripheral portion of said retort not normally occupied by said wood bed, and to direct said air peripherally of said retort in a direction opposite to the rotation of said retort, whereby said wood gas is intermixed with said air and burned to the extent permitted by the available oxygen of said air, as it flows peripherally around said retort, before the hot, oxygen-deficient gaseous products of its combustion again flow over said wood bed to entrain additional wood gas, and

f. means operable to remove the resulting charcoal from the lower end of said retort. 

1. A METHOD OF PRODUCTING CHARCOAL WOOD OR THE LIKE IN SMALL PIECES CONSISTING OF THE FOLLOWING STEPS: A. CONTINUOUSLY INTRODUCING SAID WOOD INTO THE HIGHER END OF AND ELONGATED, SLIGHTLY INCLINED RETORT WHICH IS INCLINED DOWNWARDLY TOWARD ITS OPPOSITE OR EXIT END, B. ROTATING THE RETORT ABOUT IS LONGITUDINAL AXIS, WHEREBY THE WOOD PIECES ARE CONSTANTLY TUMBLED AND ADVANCED TOWARD THE LOWER END OF THE RETORT ALONG THE LENGTH THEREOF, IN THE FORM OF A BED ONLY PARTIALLY FILLING SAID RETORT, C. HEATING SAID WOOD BY COMBUSTION OF AN EXTERNAL FUEL LONG ENOUGH ONLY TO DRY AND INITIATE CARBONIZATION OF A PORTION OF SAID WOOD, WHERBY A COMBUSTIBLE WOOD GAS IS DRIVENRTION FROM SAID WOOD, AND D. INTRODUCING AIR TANGENTIALLY INTO A PERIPHERAL PORTION OF THE RETORT NOT OCCUPIED BY THE WOOD BED, IN A DIRECTION OPPOSITE TO THE ROTATION OF THE RETORT AND AT A POINT ANGULARLY REMOTE FROM THE BED, CONSIDERED IN THE DIRECTION OF CIRCULATION OF THE AIR, WHEREBY WOOD GAS DRIVEN OFF IS ENTRAINED INN THE CIRCULATION, MIXED WITH THE AIR, AND BURNT TO THE EXTENT PERMITTED BY THE AVAILAVLE OXYGEN OF THE ADMITTED AIR, BEDFORE THE CIRCULATING GASES AGAIN ENGAGE SAID BED, THE HEAT OF SAID COMBUSTION SERVING TO DRY SUBSEQUENTLY ADDED WOOD AND BRING IT TO CARBONIZATION TEMPERTURE, AND THE TUMBLING OF THE WOOD PIECES IN THE BED SERVING TO EXPOSE THE ENTIRE MASS OF SAID BED TO THE HEAT OF SAID COMBUSTION, AND E. REMOVING THE RESULTING CHARCOAL FROM THE LOWER END OF SAID RETORT.
 2. An apparatus for producing charcoal from wood or the like in small pieces comprising: a. an elongated retort the axis of which is inclined slightly from horizontal whereby to have upper and lower ends, b. means operable to introduce said wood into the upper end of said retort at a regulated rate, c. means operable to rotate said retort about its longitudinal axis at a rate sufficiently slow that said wood gathers in a bed in the lower portion of said retort, said bed tending to rise along the ascending side of the retort until gravity causes the wood pieces to tumble, said bed also being moved along the length of said retort as a result of said tumbling and the inclination of said retort, d. means operable to burn an external fuel in or adjacent said retort long enough only to dry and bring to carbonization temperature a portion of said wood, whereby carbonization is initiated and a combustible wood gas emanates from said wood, and e. means operable to inject air in regulated amounts tangentially into a lower peripheral portion of said retort not normally occupied by said wood bed, and to direct said air peripherally of said retort in a direction opposite to the rotation of said retort, whereby said wood gas is intermixed with said air and burned to the extent permitted by the available oxygen of said air, as it flows peripherally around said retort, before the hot, oxygen-deficient gaseous products of its combustion again flow over said wood bed to entrain additional wood gas, and f. means operable to remove the resulting charcoal from the lower end of said retort. 